1. Field of the Invention
The present invention relates to a discontinuous reception mechanism and, more particularly, to a method for optimizing discontinuous reception mechanism in random access and scheduling request.
2. Description of Related Art
There are only two different Radio Resource Control (RRC) states, RRC_IDLE and RRC_CONNECTED, in Evolved Universal Terrestrial Radio Access (E-UTRA), unlike the four different RRC connected states adopted in Universal Mobile Telecommunication System (UMTS). In order to achieve better user equipment (UE) power saving while preventing unnecessary UE state transitions between RRC_IDLE and RRC_CONNECTED, Discontinuous Reception (DRX) is introduced in E-UTRA. In E-UTRA, generally the UE has to always monitor the Physical Downlink Control Channel (PDCCH), which informs the LE about the resource allocation of Paging Channel (PCH) and Downlink Shared Channel (DL-SCH), and Hybrid ARQ information related to DL-SCH and also carries the uplink scheduling grant. The introduction of DRX functionality enables UE to stop monitoring PDCCH for uplink (UL) and downlink (DL) transmission during some period of time, so that the UE only needs to be “awake” during the “active-time” duration.
In the current DRX mechanism in E-UTRA (see “E-UTRA MAC protocol specification (Release 8); 3GPP TS 36.321 V8.1.0 (2008-03); 3rd Generation Partnership Project; Technical Specification Group Radio Access Network”, which is incorporated herein for reference), the UE behaviors are mainly determined by several relevant timers and/or whether some conditions are fulfilled. The timing to start and stop such timers, and corresponding UE behaviors upon the expiration of such timers are also defined.
In view of the DRX mechanism, the UE may be configured with a DRX functionality that allows it to not continuously monitor the PDCCH. The time UE should monitor PDCCH is defined as Active Time which is mainly determined by several relevant timers and/or whether some conditions are fulfilled. Specifically, when a DRX cycle has been configured, the UE shall be operated according to the DRX functionality illustrated in the following Table 1.
TABLE 1When a DRX cycle is configured, the Active Time includes thetime while:  the On Duration Timer or the DRX Inactivity Timer or a  DRX Retransmission Timer or the Contention Resolution  Timer is running; or  a Scheduling Request is pending (as described in subclause  5.4.4); or an uplink grant for a retransmission can occur; or  a PDCCH indicating a new transmission addressed to the  C-RNTI or Temporary C-RNTI of the UE has not been received  after successful reception of a Random Access Response  (as described in subclause 5.1.4).
The Active Time defined in current DRX mechanism covers the sub-frames in which UE needs to monitor PDCCH, but there're some unnecessary overlaps between the conditions and for some cases, e.g. random access procedure and scheduling request procedure, UE monitors PDCCH in sub-frames when it does not need to, causing unnecessary UE power consumption which is defeating the purpose of DRX mechanism.
From MAC's point of view, currently there are two different Random Access (RA) procedures according to different causes of initiation of the RA procedure: one is non-contention based (e.g. initiated by a PDCCH order for the case of DL data resuming) and the other is contention based (e.g. the cases of initial access, UL data arrival triggered Buffer Status Report (BSR) which triggers RA when there's no dedicated scheduling request resource).
In contention based RA procedure, as shown in FIG. 1(A), UE first sends an RA preamble to the network (step S101) on Physical Random Access Channel (PRACH), and then UE is expected to monitor PDCCH for RA response within a TTI window after its transmission of the RA preamble (step S102). When UE transmits the first scheduled UL transmission according to the UL grant indicated in the RA response (step S103), it is expected to monitor PDCCH for contention resolution until it receives the message for contention resolution (step 104), or until the expiration of the Contention Resolution Timer.
Back off Indicator can be included in RA response as FIG. 1(D) shows. The BI field indicates the back off parameter value. Upon receiving RA response with BI, UE stores the value as back off parameter. When receiving RA response without BI, the value stored in the UE is set to 0. The value is initiated as 0. When UE applies back off, it delays the subsequent preamble transmission by the back off time which is computed by the UE base on the stored back off parameter.
If the UE does not receive its RA response within TTI window after it transmits preamble, as shown in FIG. 1(B), it will apply back off (step S105) and start another round of RA attempted to send RA preamble at next available PRACH resource. Alternatively, if the UE finds itself failed the contention resolution either because of no contention resolution message is received before the expiration of Contention Resolution Timer or because of the contention resolution message contains Contention Resolution Identity for other UE, as shown in FIG. 1(C), it will apply back off and start another round of RA attempted to send RA preamble at next available PRACH resource.
While for non-contention based RA procedure, as shown in FIG. 2, UE first receives an RA preamble assignment from the network (step S201) and sends an RA preamble as assigned to the network (step S202). Then, UE is expected to monitor PDCCH for RA response within a TTI window after its transmission of the preamble (step S203).
According to current DRX mechanism, when UE is in the DRX mode, UE will keep on monitoring PDCCH when “a PDCCH addressed to the C-RNTI or Temporary C-RNTI of the UE has not been received after successful reception of a Random Access Response”. It is valid for both contention based and non-contention based RA procedure. However for contention based RA procedure, as shown in FIG. 3(A), after successfully receiving its RA response, UE still keeps on monitoring PDCCH as PDCCH addressed to the C-RNTI or Temporary C-RNTI of the UE is not received, which will cause unnecessary UE battery consumption as UE is only necessary to monitor PDCCH when the contention resolution timer is running. Further, as shown in FIG. 3(B), if the no contention resolution message is received before the expiration of Contention Resolution Timer, UE still keeps on monitoring PDCCH when applying back off and waiting for next available PRACH resource as PDCCH addressed to the C-RNTI or Temporary C-RNTI of the UE is not received, which also will cause unnecessary UE battery consumption. In addition, as shown in FIG. 3(C), when no contention resolution message is received before the expiration of Contention Resolution Timer, at the next RA procedure round, UE still keeps on monitoring PDCCH before the TTI window is started, and also if no RA response is received within the TTI window, UE still keeps on monitoring PDCCH when applying back off and waiting for next available PRACH resource as PDCCH addressed to the C-RNTI or Temporary C-RNTI of the UE is not received, which will cause unnecessary UE battery consumption.
Another aspect is related to the Scheduling Request (SR) pending when UE is in DRX mode. A scheduling request is triggered in UE whenever a Regular Buffer Status Report (BSR) needs to be sent from UE but no UL resources is allocated for new transmission to the UE for this TTI, and is considered pending until UL resources are granted for a new transmission. As shown in FIG. 4, after a scheduling request is triggered (step S401), if there is a dedicated SR resource configured for UE, UE will send the SR on the resource to inform network the presence of the BSR (step 402) when the SR is pending until UL resources are granted for a new transmission (step S403). However if there is no dedicated SR resource configured for UE, as shown in FIG. 5, after a scheduling request is triggered (step 501), UE will initialize an RA procedure in order to transmit the BSR (step 502), in which SR is considered pending until UL resource (RA response) is granted for a new transmission (step 503).
As shown in FIGS. 4 and 5, a triggered SR is considered pending until UL resources are granted for a new transmission, but such condition will cover some subframes in which UE doesn't need to monitor PDCCH if RA procedure is initiated when there's no dedicated SR resource which is configured for the UE to send SR. FIG. 6 shows the UE behavior according to current specification before RA response is received: the SR is considered pending so that the UE will keep on awake to monitor PDCCH, but in fact after the TTI window, UE would take some time decoding the PDCCH for the last TTI of the window and decided no response is received, and it will apply back off. After back off is performed, it will wait for next available PRACH resource for preamble re-attempt, and some time, e.g. 2 ms in FIG. 6 after preamble transmission, which is the time for the network to process the preamble and generate the RA response, it will start the TTI window. So during the time covered by SR being pending, there's a lot of time UE need not monitor PDCCH.
Therefore, it is desirable to provide an improved method for optimizing discontinuous reception in random access and scheduling request to mitigate and/or obviate the aforementioned problems.